The overall aim of this study is to conduct a rigorous investigation into the relative advantages and disadvantages of the use of extraction strategies versus non-extraction strategies in the orthodontic treatment of adult patients with mild to moderately severe Angle Class I and Class II malocclusions. For the purposes of this study, adult patients are defined as individuals who commence active orthodontic treatment after their twentieth birthday. The study will have both a retrospective component and a prospective component and will constitute an interinstitutional collaboration between the Department of Growth and Development of the University of California, San Francisco and the Department of Orthodontics of the University of the Pacific. During the initial five year grant period, we will seek the characterize and compare the effects of adult extraction and non- extraction strategies in two main areas: 1. Morphologic changes associated with treatment, including (a) Changes during treatment in face form, arch form, tooth alignment and tooth intercuspation, and (b) Post-treatment stability of the changes noted in (a), and 2. Physiological costs and benefits of treatment, particularly in terms of periodontal pathology. We will commence and complete the retrospective component of the study and the active phase of treatment for the first cohort of subjects in the prospective component. The retrospective component will examine a representative group of subjects treated earlier by expert orthodontists. The prospective component will include a randomized clinical trial of new design with provision for clinician-preferred treatment.